Polypeptide hormones bind to cell-surface receptors and generate second messenger molecules that regulate the cellular metabolism. In addition, the receptor-ligand complex is internalized by receptor-mediated endocytosis and localized within endosomes and lysomes in the cytoplasm of the cell. We postulated that the internalized ligand may have an intracellular role and tested this idea by directly injecting cytokines into the cytoplasmic compartment. Microinjection of interferon-gamma (IFNGamma) was shown to induce Ia expression across the species barrier and injection of TNFAlpha induced rapid cytotoxicity and DNA fragmentation. Recently, injection was used to identify a role for PKC in the signal transduction leading to surface Ia expression on macrophages, a biological response seen normally from IFNGamma treatment.